Fatigue cracking and partial discharge (PD) events are both issues for electric generator systems. The phenomena are related in that each may be a contributing factor of the other. Partial discharges tend to increase the number of fatigue cracks, and an increase in the number of fatigue cracks tends to increase the frequency of partial discharges. A partial discharge is a localized dielectric breakdown of an electrical insulation system that does not bridge the space between two conductors. A partial discharge generates high frequency transient current pulses that persist for a time period in the range of nanoseconds up to a microsecond. Partial discharges often begin within voids, cracks, such as fatigue cracks, or inclusions, or at conductor-dielectric interfaces within a solid dielectric. A partial discharge may also occur along the boundary between different insulating materials. Although fatigue cracking may result from any type of cyclic stresses on a material, including both mechanical and electrical stresses, the stresses are primarily electrical in the context of electric generator systems. Cumulative partial discharges may cause fatigue cracking events.
Partial discharges cause progressive deterioration of insulating materials, ultimately leading to an electrical breakdown. Repetitive PD events cause irreversible mechanical and chemical deterioration of the insulating material. Damage is caused by the energy dissipated by high energy electrons or ions, ultraviolet light from the discharges, ozone attacking the void walls, and cracking as the chemical breakdown processes liberate gases at high pressure. There are three main types of partial discharges: internal partial discharges, surface partial discharges, and corona partial discharges. A history of internal PD in a high-voltage system eventually triggers electrical treeing. A history of surface PD eventually induces insulation tracking. A history of corona PD originating in the high-voltage connections, however, is generally not harmful.
PD monitoring and detection involves evaluating the dielectric condition of a system by monitoring electrical signals and an analysis of arcing, electric field, materials, wave propagation and attenuation, sensor spatial sensitivity, frequency response and calibration, noise, and data interpretation. An electric arc is a visible plasma discharge between two electrodes that is caused by electrical current ionizing gases in the air.
The magnitude of a partial discharge is related to the extent of damaging discharges occurring and therefore is related to the amount of damage being inflicted on the insulation. The pulse repetition rate indicates the quantity of discharges occurring at the various maximum magnitude levels.
A partial discharge measurement system may include an ultra-high frequency (UHF) sensor, a high frequency current transformer (HFCT), an ultrasonic microphone, a transient earth voltage (TEV) sensor or coupling capacitor, a phase-resolved analysis system to evaluate a wide range of signal frequencies, or combinations thereof. A UHF sensor generally detects in the range of 300 MHz-1.5 GHz. An HFCT generally detects in the range of 500 kHz-50 MHz. An ultrasonic microphone generally detects in a range around 40 kHz. A TEV sensor or coupling capacitor generally detects in the range of 3 MHz-100 MHz. A phase-resolved analysis system compares pulse timing to alternating current (AC) frequency.
A significant problem with electrical detection of partial discharges is that the detection equipment is highly susceptible to electromagnetic noise, which can lead to false detection of partial discharges, typically as false positives. In high voltage systems, however, the spike caused by a partial discharge may be very small compared to the amplitude of the system voltage and may be missed, leading to a false negative. Usually the equipment subjected to testing also needs to be taken off-line, energized from a high-voltage source, and then tested, which requires a lot of time and equipment. Conventional PD measurement systems cannot detect fatigue cracks and may detect but do not locate the PD events.
An acoustic emission (AE) is a transient elastic wave within a material, typically the result of a rapid release of localized stress energy, such as when a material undergoes an irreversible change in its structure. Fatigue cracking events generate acoustic emissions.
Acoustic detection systems detect acoustic emissions as sound waves generally in the frequency range of 20 kHz up to more than 1 MHz. Acoustic detection systems may be external or internal systems. External acoustic detection systems employ sensors mounted to the outside of the equipment to be monitored. Internal systems, on the other hand, use sensors placed inside of the equipment to be monitored to directly measure the pressure wave.
An important advantage of acoustic detection over other methods is that positioning information is available by using sensors at multiple locations. This position information may help to identify the location of the source of the AE. Another advantage of acoustic detection is its immunity to electromagnetic interference or electromagnetic noise.